METAL FORMING

PROCESSES

J. J. Ellard, PhD

MEFP-520 : 2024/2025
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Fundamentals of Metal
Forming

Part B

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 Contents
3. Effect of Temperature, Strain Rate and
Metallurgical Structure on Metal Working
4. Friction and Lubrication
5. Residual Stresses
6. Workability
7. Deformation Zone Geometry

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Strain Rate on Metal Forming
➢Theoretically, a metal in hot working behaves like a perfectly
plastic material, with strain hardening exponent n = 0
• The metal should continue to flow at the same flow stress, once that
stress is reached

➢However, an additional phenomenon occurs during

deformation, especially at elevated temperature: flow stress
depends on strain rate
• Strain rate sensitivity - as strain rate increases, resistance to
deformation increases.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Strain Rate on Metal Forming
➢Strain rate in forming is directly related to speed of
deformation, ν.
➢Deformation speed ν = velocity of the ram or other
movement of the equipment.
➢Strain rate is defined:
ν
𝜀ሶ =
ℎ
where 𝜀ሶ is true strain rate, ℎ is instantaneous
height of workpiece being deformed

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Strain Rate on Metal Forming
➢High deformation speed (high strain rate) leads to:
• High flow stress
• Increased temperature of the workpiece
• Improved lubrication at the tool-metal interface
➢Too high deformation speed can cause metal cracking,
plastic instability in cold working, and hot shortness in
hot working.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Strain Rate on Metal Forming
Effect of strain rate on Same relationship
flow stress at an elevated plotted on log-log
work temperature coordinates

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Strain Rate on Metal Forming

➢ Strain rate Sensitivity Equation:

Y f = C ε m
where C = strength constant (analogous but not
equal to strength coefficient in flow curve
equation)
m = strain-rate sensitivity exponent

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Metallurgical Structure on Metal Forming
➢The presence of preferred orientation
causes anisotropy of mechanical
properties, especially in rolled sheets.
➢The development of texture is the
formation of deformation bands or
shear bands, which are regions of
distortion where a portion of grains has
rotated towards another orientation to
accommodate the applied strain.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Metallurgical Structure on Metal Forming
Examples: Plastic working in two-phase alloys
➢The plastic working characteristics of two-phase alloys depends
on the microscopic distribution of the two phases.
➢A high vol. fraction of hard uniformly dispersed particles
increases the flow stress and makes working difficult.
• Hard and Massive particles tend to
fracture on deformation with softer
matrix.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Metallurgical Structure on Metal Forming
➢Second phase particles or
inclusions will be distorted in
the principal working
direction (fibrous structure) –
affect mechanical properties.
➢Precipitation hardening during
hot working results in high
flow stress and lowered
ductility

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 4. Friction and Lubrication on Metal Forming
➢Friction at tool-workpiece interface depends on
geometry of the tooling and the geometry of the
deformation, temperature, nature of metal, speed of
deformation.

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 4. Friction and Lubrication on Metal Forming
➢When two surfaces are brought into contact, the high
spot (asperities) will come into contact.

➢As we increase the load, the metal at the asperities

deform plastically and produce sub-shear zone.

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 4. Friction and Lubrication on Metal Forming
➢ The coefficient of friction is given by:

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 4. Friction and Lubrication on Metal Forming
Examples: Homogeneous compression of a flat
circular disk
Assumptions: No barreling and small thickness, then the
frictional conditions on the top and bottom faces of the disk are
described by a constant coefficient of Coulomb friction:

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 4. Friction and Lubrication on Metal Forming
Examples: Friction in Forging

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4. Friction and Lubrication on Metal Forming

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 5. Residual Stresses
➢ Residual stresses are generated by non-uniform plastic deformation when
external stresses are removed.
• For example, in rolling process, the surface grains in the sheet are
deformed and tend to elongate while the grains in the centre are
unaffected.
• Due to continuity of the sheet, the central fibres tend to restrain the
surface fibres from elongating while the surface fibres tend to stretch the
central fibres.
• This leads to the residual stress pattern consisting of high compressive
stress at the surface and tensile stress in the centre.

resulting distribution of
Inhomogeneous longitudinal residual
deformation in stress over thickness of
rolling of sheet sheet
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 5. Residual Stresses
➢Residual stresses are only elastic stresses - the maximum value
which a residual stress can reach is the yield stress of the material.
➢Residual stresses can be considered the same as ordinary applied
stresses.
➢Compressive residual stress can effectively subtract from the
applied tensile stress.
➢Metals containing residual stresses can be stress relieved by
heating to a temperature where the yield strength of the material
is the same or lower than the value of the residual stress such that
the material can deform and release stress.
➢However, slow cooling is required otherwise residual
stress can again develop during cooling.
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 6. Workability
➢This is concerned with the extent to which a material can be
deformed in a specific metal working processes without the
formation of cracks.
➢Cracks which occur in metal working processes can be grouped
into three broad categories:

Examples of cracks in metalworking

a) Free surface crack.
(a) (b) b) Surface crack from heavy die friction in extrusion.
c) Centre burst or chevron cracks in a drawn rod.
(c)
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 7. Deformation zone Geometry
➢Only one part of the material is being deformed at any one time.
➢This part of the material being deformed is called a deformation
zone.
➢This zone includes:
• The interfaces with the tools/dies.
• The internal interfaces with the material that is not plastically
deforming.
➢The deformation zone geometry is characterised by a ∆ factor.

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 7. Deformation zone Geometry
➢Because the metal is moving relative to the tooling (die), there
will be friction at that interface.
➢At, or in the vicinity of, the interface between the (plastically)
deforming and undeforming metal there will have to be ‘extra’
deformation to maintain material continuity.

• Inside the deformation zone the

deformation corresponds to the ideal
deformation (compression).
• Near the ‘interface’ there are additional
shears: inhomogeneous deformation.
• The extra plastic work associated with that
deformation is called redundant work

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7. Deformation zone Geometry

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7. Deformation zone Geometry

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